Freeze-drying apparatus



1962 D. A. COPSON ET AL FREEZE-DRYING APPARATUS Filed April 27, 1959OSC/LLATO REFRIGERA T/0N 0 0000000 0000 on o ouoooo 0o P HA m K N sw aw: n ,0 A R k Z. p0 Y/ My 3 31,048,928 Patented Aug. 14, 1962 Filed Apr.27, 1959, Ser. No. 809,304 3 Claims. (Cl. 341) This invention relates tothe use of microwave energy for the freeze-drying of material such asfoods.

Drying by sublimation of such materials as foods, tissues and culturesof organisms that have first been frozen has proved to be a usefulpreservation process particularly for heat l-abile substances. However,many materials are not amenable to this treatment because of theirdimensions or structures. For example, as beefsteak or bone tissue isfreeze-dried, the highest surface at which sublimation occurs steadilyrecedes behind a dried portion of the substance that acts as a poorconductor for "heat. As the process goes on, more and more heat isdissipated in the dried portion of the material before reaching thevaporizing surface to cause sublimation.

An improved technique over the application of heating utilizes the factthat many materials in the dry state absorb little electromagneticenergy at radio or microwave frequencies compared to the energy absorbedby the same material in a moist frozen state. The result is thatmicrowave energy applied to moist frozen material is absorbed almostentirely by the moist frozen portion of the material. By this process,little or no energy is lost in transmission through the dried material,making the drying process far more efiicient. In fact, a given sizedpiece of beefsteak can be dried by microwave energy in about one-thirdof the time required to dry it in other forms of heating under the sameconditions.

It is important that the sublimed water be removed from the place wherethe heat of sublimation is applied. Heretofore it has been considerednecessary to physically separate the place of sublimation from a placeof desublimation where refrigeration is provided so that the condensatewill not flow back to the place of sublimation and to pump the moisturein vapor form from the place of sublimation to the place ofdesublimation where the moisture is collected thereby effectingdehydration of the material being processed. However, the expansion ofwater in its sublimation from ice is several orders of mag ni-tude,i.e., one pound of water at 1 mm. pressure occupies a volume of about14,300 cu. ft. This substantial increase in volume has heretoforerequired the use of high capacity motors and pumps and suction pipelines of large dimensions even when moderate quantities of water are tobe removed. Further the efliciency of freeze-drying systems depends to alarge extent, particularly during the initial drying phase, on theunimpeded flow of vapor from the region of sublimation or high vaporpressure to the region of low vapor pressure or the desublimator. Aspointed out heretofore the place where the heat of sublimation isapplied was separated from the desublimator where refrigeration occurs,so that the condensate will not flow back into the system.

The present invention contemplates a single vacuum chamber enclosing anRF still constructed in the form of an RF choke and comprised of screenmaterial opaque to radio and microwave frequency energy while providinga large number of passages in combination with coils or the likecontaining a refrigerant surrounding and enclosing the RF still.Conventional means are provided to supply microwave energy to theinterior of the still, lower the temperature of the condenser coils toabout --30 to 40 C. and evacuate the vacuum chamber to about 0.5-1 mm.pressure. Upon reduction of the temperature of the coils and thepressure in the chamber to the desired operating temperature andpressure, and provision of microwave energy to the still or containercontaining the frozen material to be treated, water in the frozenmaterial is sublimated and collected on the coils and noncondensablegases are eliminated by the vacuum pump. Due to the location of thecondenser coils in close proximity to the frozen material being dried,the large area of cold surface provided by the condenser coils and thelarge number of passages in the radio frequency screen, there isprovided at all times a highly efficient and very short andsubstantially unimpeded vapor path from the region of high vaporpressure in the still to the low vapor pressure at the condenser coilsthereby greatly increasing the efiiciency of freeze-drying systems andsimplifying their construction and manufacture.

Other and further features and advantages of the invention will becomeapparent from the following description taken in connection with theaccompanying drawing illustrating the invention which shows aperspective view with parts broken away of an apparatus for carrying outthe invention.

In the drawing the reference numeral 10 designates a sealed container orevacuation chamber having a sealable access door 11. Supported withinthe container as by legs 12 is a second conductive container or heatingchamber 13 containing an access door 14. The heating chamber '13 forms acavity to which propagated electro magnetic radio frequency energy isapplied from an oscillator 15 through a transmission line 16 of the waveguide type such as shown, for example, in Patent No. 2,540,036 issuedJanuary 30, 1951 to P. L. Spencer. Frequencies which are especiallysignificant for this purpose are those in the microwave range, which maybe considered as those lying between 300 megacycles per second and30,000 megacycles per second. The two most practical frequencies in thisrange are in the region of 2450 megacycles per second and 915 megacyclesper second. In accordance with the invention the heating chamber 13 isadapted to prevent the propagation of electromagnetic energy from itsinterior to the container 10 while permitting atmospheric communicationthere-between. This may be achieved by forming the walls 17 of theheating chamber 13 with openings 18 of a diameter small with respect tothe wave length of the radio frequency energy to prevent the propagationof the radio frequency energy from the interior of chamber 13 whilepermitting atmospheric communication. Disposed in close proximity to andsubstantially enclosing the walls 17 of the heating chamber 13 arecondenser coils 19. Condenser coils 19 are connected to suitablerefrigeration means 21 adapted to lower the temperature of the condensercoils 19 to about 30 to 40 C. A pipe 22 provides communication betweenthe interior of container 10 and a pump 23. Material 24 in the frozenstate to be dried is supported on a centrally located platform 25 ofdielectric material formed with suitable openings or slots to permit thefree passage of the vapor. The access door 14 is of course closed toprevent leakage of radio frequency energy. As radio frequency energypenetrates the frozen material 24, the ice or other component to bevaporized evaporates without first becoming a liquid, that is itsublim-ates due to the absorption of the radio frequencyenergy-producing heat. This sublimation produces a region of driedmaterial, represented by the section 24a, and leaves a core of frozenundried material, represented by the section 24b. The dried material 24aoffers no appreciable impedance thus permitting the sublimation of themoist material to continue at a substantially uni-form rate. The onlyprecaution that need be taken is to keep the radio frequency energy frombeing applied fast enough to raise the temperature of any portion of thematerial above the melting point of the vaporizable portion of thematerial. Should this be permitted to happen, the liquid, particularlyif water, would offer considerably greater impedance to the radiofrequency energy and hot spots might develop, which in the case of food,would cause changes affecting the flavor. However, known methods formaintaining the voltage supplied to the oscillator at a predeterminedvalue will accomplish this objective.

The present invention may be used to evaporate out from any material anycomponent that may be evaporated from the solid state. Upon closure ofdoor 11 the evacuation of chamber by pump 23 serves to reduce thepartial pressure of the component to be evaporated in the atmosphere ofthe heating chamber 13. The heating of the material 24 provides a highvapor pressure at the material 24 and the cooling of the condenser coils19 provide a large low temperature region in close proximity to thematerial 24. This, in combination with the heating chamber 13 havingwalls 17 effectively comprising microwave energy chokes, provides at alltimes a substantially more efficient, very short and unimpeded vaporpath from the region of high vapor pressure or material 24 to the lowvapor pressure region or condenser coils 19 which rapidly condense andcollect in frozen form the vapor sublimated from the material 24 in theheating chamber 13.

Vapor condensed on the coils 19 may be removed during off cycles of thestill by raising the temperature of the coils 19 to a point abovefreezing and removing the vapor as it forms with the pump or othersuitable means. This process may be accelerated if the refrigerationmeans 211 is reversible, i.e., the coils can be heated.

This invention is not limited to the particular details of constructionand materials described, as many equivalents will suggest themselves tothose skilled in the art. It is accordingly desired that the appendedclaims be given a broad interpretation commensurate with the scope ofthe invention within the art.

What is claimed is:

1. Apparatus for removing a vaporizable component by sublimation frommaterial including a solid and said vaporizable component in the solidstate comprising: first container means for providing an atmosphere at apressure substantially lower than atmospheric pressure; pump meanscommunicating with said first container means for maintaining said lowerpressure; second container means disposed in said first container meansenclosing said material to be treated, a substantial portion of saidsecond container means containing a plurality of passages forming amicrowave energy choke for preventing the propagation of microwaveenergy from the interior of said second container means to the interiorof said first container means while permitting substantial atmosphericcommunication therebetween; a microwave electrical energy generator; apropagated electromagnetic wave-guiding structure coupled to saidgenerator and communicating with said second container means forapplying propagated electromagnetic microwave energy to said material tosupply the heat of sublimation to sublimate a substantial portion ofsaid component; condenser means having a substantial area substantiallyenclosing and adjacent said portion of said second container meansforming a microwave energy choke for at least assisting in causing saidsublimed component to pass outwardly through said choke portion andthereafter to condense on said condenser means; and means formaintaining said condenser means at a temperature substantially lowerthan freezing whereby there is provided a short substantially unimpededvapor path between said material and said condenser means and saidsublimed component is efiiciently condensed on said condenser means.

2. In combination, means for generating electromagnetic energy having afrequency in the microwave region of the spectrum, a chamber forreceiving a frozen aqueous material, waveguide means connected betweensaid generating means and said chamber for coupling said electromagneticenergy to said chamber to sublimate a substantial portion of the waterin said material, a sealed container for enclosing said chamber, meansfor maintaining the pressure within said sealed container at a valuesubstantially lower than atmospheric pressure, a plurality of aperturesin a portion of the walls of said chamber for preventing the propagationof said electromagnetic energy from said chamber to said sealedcontainer and for providing atmospheric communication therebetween, andmeans for condensing water sublimated from said material.

3. In combination, means for generating electromagnetic energy having afrequency in the microwave region of the spectrum, a chamber forreceiving a frozen aqueous material, waveguide means connected betweensaid gencrating means and said chamber for coupling said electromagneticenergy to said chamber to sublimate a substantial portion of the waterin said material, a sealed container for enclosing said chamber, meansfor maintaining the pressure within said sealed container at a valuesubstantially lower than atmospheric pressure, a plurality of aperturesin a portion of the walls of said chamber for preventing the propagationof said electromagnetic energy from said chamber to said sealedcontainer and for providing atmospheric communication therebetween,condenser means having a substantial area adjacent to and enclosing asubstantial portion of said chamber for condensing in frozen form watersublimated from said material, and means for maintaining said condensermeans at a low temperature.

References Cited in the file of this patent UNITED STATES PATENTS2,345,204 Lodwig Mar. 28, 1944 2,513,991 Bradbury July 4, 1950 2,585,825Nyrop Feb. 12, 1952 2,859,534 Copson Nov. 11, 1958

